Does the speed of light vary in non-inertial frames? The speed of light is the same in all inertial frames.
Does it change from a non-inertial frame to another? Can it be zero?
If it is not constant in non-inertial frames, is it still bounded from above? 
 A: To elaborate on Mark M's answer:
If you consider an accelerating reference frame with respect to Rindler coordinates (where time is measured by idealized point-particle accelerating clocks, and objects at different locations accelerate at different rates in order to preserve proper lengths in the momentarily comoving reference frames), then light may not move at c, and can in fact even stop.
Specifically, for motion in one dimension, consider the transformations in natural units ($c=1$) between cartesian co-ordinates $(t,x)$ to Rindler co-ordinates $(t_R, x_R)$, for an observer accelerating at a rate of $g$ from an initial position $x_I = 1$, in order to maintain a fixed interval from the origin:
$$\begin{align*}
   t_R &= \tfrac{1}{g}\mathop{\mathrm{arctanh}}\left(\frac{t}{x}\right) \;,
&
   x_R &= \sqrt{x^2 - t^2\,}\;;
\tag{C $\to$ R}
\\[2ex]
   t &= x_R \sinh(gt_R) \;,
&
   x &= x_R \cosh(gt_R) \;.
\tag{R $\to$ C}
\end{align*}$$
A light signal emitted from some initial position $x_\varphi$ along the X-axis follows the trajectory $x = x_\varphi + vt$, where $v = \pm 1$ just gives the direction. Consider the trajectory that it follows in Rindler co-ordinates:
$$\begin{align*}
  x_R^2 = x^2 - t^2 &= (x_\varphi + vt)^2 - t^2
\\
 &= x_\varphi^2 + 2x_\varphi vt \tag{as $v^2t^2 - t^2 = 0$}
\\
 &= x_\varphi^2 + 2x_\varphi vx_r \sinh(gt_R)\;;
\end{align*}$$
using the quadratic formula, we obtain
$$\begin{align*}
x_R &= 
x_\varphi\Bigl[v\,\sinh(gt_R) + \cosh(gt_R)\Bigr]
\;=\; x_\varphi\exp(\pm gt_R) \;, \quad\text{for $v = \pm 1$}.
\end{align*}$$
Yes, that's an exponential function on the right. 
It follows that the speed of a light signal is dependent on position in Rindler co-ordinates: the speed of the light signal emitted at $t = 0$ at $x_\varphi$ is
$$ \frac{\mathrm dx_R}{\mathrm dt_R} = \pm gx_\varphi \exp(\pm g \cdot 0) = \pm g x_\varphi \;.$$
We can show that the speed of light is a function only of position as follows.
A light emission (to either the left or right) from $x_1 > 0$ at $t_1 = 0$ reaches a position $x_2 = x_1 \exp(\pm g t_2)$ after an elapsed time of $t_2$; its speed at that time is $v_{1\to2} = g x_1 \exp(\pm g t_2)$, which is equal to the instantaneous speed of a light signal sent from the position $x_2$ at time $t_1 = 0$. So in natural units, the speed of light in Rindler co-ordinates is
$$ c(x) = gx \quad\Bigl[\text{in non-natural units,}\;\; gx/c\Bigr],$$
where $x$ is the location of the light signal.
In particular, any light signal appears to travel at the inertial constant speed $c$ just as it passes them. 
This has a few consequences. Light signals sent from positions $0 < x_\varphi < x_I = 1$ will move more slowly in the proper time of the Rindler observer, with light signals moving to the right taking longer than usual to catch up to the accelerating observer, up until it reaches them, at which point it seems to travel at $c$. As we take $x_\varphi \to 0$, light signals in any direction appear to slow to a stop. Such beams of light define the Rindler horizon of the reference frame, cutting away a region of space-time from which the observer cannot obtain any information because they see the objects in it accelerating away too quickly, as with the event horizon of a black hole. Conversely, light signals at positions $x_\varphi > x_I = 1$ may appear to be travelling faster than c.
A: The speed of light has a velocity of c in an accelerating frame of reference if you constrain yourself to making local measurements. So, the simple answer is that yes, the speed of light remains constant.  However, if you don't take purely local measurements, you can get a different speed depending on your coordinate system.  If you use a coordinate system where you, an accelerating observer, are at rest (like Rindler coordinates, where time is measured by accelerating clocks and distance is measured by rulers undergoing Born rigid acceleration) then light may not move at c.
